Germanium has much higher electron and hole mobility than silicon, therefore it may be a possible replacement for silicon as used in high speed CMOS devices, Ritenour et al., Epitaxial Strained Germanium p-MOSFETs with HfO2 Gate Dielectric and TaN Gate Electrode, International Electron Devices Meeting Technical Digest, 2003, Washington, D.C.; Chui et al, A Germanium NMOSFET Process Integrating Metal Gate and Improved Hi-k Dielectrics, International Electron Devices Meeting Technical Digest, 2003, Washington, D.C.; Low et al., Germanium MOS: An Evaluation from Carrier Quantization and Tunneling Current, 2003 Symposium on VLSI Technology Digest, pp 117–118; and Bai et al., Ge MOS Characteristics with CVD HfO2 Gate Dielectrics and TaN Gate Electrode, 2003 Symposium on VLSI Technology Digest, pp 121–122.
Pure germanium, grown directly on silicon, is the best candidate for near-infrared photodetectors because of its compatibility with silicon technology, and its high absorption in near-infrared wavelengths, up to 1.55 μm. It has potential applications in low-cost monolithic transceivers for optical communications, Colace et al., Efficient high-speed near-infrared Ge photodectors integrated on Silicon substrates, App Phy Let. 76, pp 1231–1233 (2000); Famà et al., High performance germanium-on-silicon detectors for optical communications, App Phy Let. 81, pp 586–588 (2002); and Hartmann et al., Reduced pressure-chemical vapor deposition of Ge thick layers on Silicon (001) for 1.3–1.55-μm photodetection, Journal of Applied Physics 95, pp 5905–5913 (2004). Because of the process compatibility with state-of-the-art silicon fabrication processes, it is possible to integrate high-speed devices containing germanium-based photodetectors into silicon ICs many applications.
However, because of the large lattice mismatch between germanium and silicon (4.2%), it is not easy to form germanium films on silicon which have the requisite flatness and low defect density required for high speed devices. Ritenour et al., supra, reported their work of epitaxial strained germanium p-MOSFET by growing thin germanium layer on thick relaxed SiGe buffer layer. Hofmann was able to grow a 1-μm thick relaxed germanium layer by surfactant-mediated epitaxy on (111) silicon, Hofmann et al., Surfactant-grown low-doped germanium layers on silicon with high electron mobilities, Thin Solid Films 321, pp 125–130 (1998). Luan et al. reported a technique to deposit a germanium epilayer on single crystal silicon by first depositing at 350° C. and then at 600° C., Luan et al., High-quality Ge epilayers on Silicon with low threading-dislocation densities, App Phy Let. 75, pp 2909–2911 (1999), and U.S. Pat. No. 6,635,110 B1, to Luan et al., granted Oct. 21, 2003, for Cycle thermal anneal for dislocation reduction. This two-step process was also reported in U.S. Pat. No. 6,537,370 B 1, to Hernandez et al., granted Mar. 25, 2003, for Process for obtaining a layer of single-crystal germanium on a substrate of single-crystal silicon, and products obtained. Luan et al, supra, also reported the technique of cyclic annealing to reduce the defect density of the germanium film. Using similar technique, several groups have reported the application in near-infrared germanium photodetectors Colace et al., Famà et al., and Hartmann et al., supra. We have also demonstrated the advantage of cyclic annealing. After cyclic annealing, the defects are reduced and concentrated near the Ge/Si interface.
Threading dislocation density of germanium film is expected to be reduced significantly by cyclic annealing, as described in the works of Colace et al., Famà et al., and Hartmann et al., supra; and by Luan et al. and Hernandez et al., supra, by the technique of forming threading dislocation at the sample edge. In small etched mesa structures, the average threading dislocation density decreased with the decrease of mesa sidewall width, Luan et al., supra. Here, the mesa sidewall served as a dislocation sink.
However, cavity formation in germanium film as a result of cyclic annealing has been detected. This undesired phenomenon can deteriorate the surface smoothness, affect the device performance, and increase the leakage current. A method is to needed to prevent cavity formation during cyclic annealing, and to prepare a smooth film after annealing at a temperature close to the melting point of germanium.
The absorption of bulk germanium falls rapidly beyond the direct band gap of 0.8 eV, corresponding to 1.55 μm. Previous works, cited herein, demonstrate that CVD deposited germanium films, if processed through heating and cooling steps, exhibited a tensile strain of 0.21% in the germanium film due to the difference of thermal expansion coefficient between germanium and silicon. Germanium band gap was reduced to 0.768 eV with this strain, and the absorption spectrum was pushed to 1.610 μm, Cannon et al., Tensile strained epitaxial Ge films on Silicon (100) substrates with potential application in L-band telecommunications, App Phy Let. 84, pp 906–908 (2004). The germanium band-gap was further reduced to 0.773 eV and 0.765 eV after silicidation at the backside of the wafer, corresponding to 1.620 μm, Liu et al., Silicidation-induced band gap shrinkage in Ge epitaxial films on Silicon, Appl Phys Let. 84, p660 (2004). A simpler method which does not require a silicidation step, is preferable because silicidation frequently results in metal contamination during the front-end processes.